To measure the effects of nitroglycerin during exercise on animals with coronary occlusive lesions, we used radioactive microspheres to assess coronary blood flow at rest and during exercise before and after nitroglycerin in dogs with surgically induced obstructive coronary disease. One region of myocardium, the endocardial portion of the zone perfused exclusively by collateral channels, showed evidence of deficient blood supply. This region had either no change or possibly a slight increase in coronary blood flow during exercise after nitroglycerin despite a fall in perfusion pressure. Thus our data suggest a nitroglycerin-induced salutary influence on coronary collateral function during exercise.